Overview
This lecture covers the distribution of water in the human body, details the major fluid compartments, and explains their compositions, boundaries, and measurement methods.
Body Fluid Compartments
- The human body is about 50–70% water, varying with age and body fat.
- Total body water is divided into intracellular fluid (ICF) and extracellular fluid (ECF).
- ICF is the fluid inside all cells, separated from ECF by the cell membrane.
- ICF makes up about two-thirds of total body water; ECF is one-third.
- If total body water is 60% of body weight, 40% is ICF, and 20% is ECF (60-40-20 rule).
- ECF is divided into interstitial fluid (outside vessels) and plasma (inside vessels).
- Interstitial fluid is about three-quarters of ECF; plasma is one-quarter.
- Interstitial fluid and plasma are separated by the capillary membrane.
Special Compartments
- Blood contains both plasma (ECF) and cells (ICF), with RBC fluid as ICF.
- Hematocrit is the fraction of blood that is RBCs.
- Transcellular fluid (e.g., pericardial, peritoneal, and cerebrospinal fluid) is a minor ECF component.
Solute Composition and Membrane Properties
- Capillary membrane allows most solutes, except proteins, to pass; proteins remain in plasma.
- Interstitial fluid has much less protein than plasma.
- ECF's main cation is sodium; main anions are chloride and bicarbonate.
- ICF's main cations are potassium and magnesium; main anions are organic phosphates and proteins.
- Both ICF and ECF are electrically neutral.
Measurement of Fluid Volumes
- Total body water = ICF + ECF; ECF = interstitial fluid + plasma volume.
- Indicator-dilution principle measures compartment volumes by injecting an indicator and assessing its dilution.
- Different indicators: TBW (crosses cell membranes), ECF (does not cross cell membranes), plasma (does not cross capillary membrane or enter RBCs).
- ICF and interstitial fluid volumes are calculated indirectly, not measured directly.
- Blood volume = plasma volume / (1 - hematocrit).
Fluid Equilibrium and Osmosis
- Cell membrane is semi-permeable: water crosses freely, most solutes do not.
- ECF and ICF osmolarity must be equal; water moves by osmosis to maintain equilibrium.
- Osmosis corrects any imbalance in osmolarity between compartments.
Key Terms & Definitions
- Intracellular Fluid (ICF) — fluid inside cells.
- Extracellular Fluid (ECF) — fluid outside cells, includes interstitial fluid and plasma.
- Plasma — fluid in blood vessels, part of ECF.
- Interstitial Fluid — fluid outside vessels, part of ECF.
- Transcellular Fluid — specialized fluids in body cavities, minor ECF component.
- Hematocrit — fraction of blood volume occupied by red blood cells.
- Indicator-Dilution Principle — method to measure fluid compartment volume using a tracer substance.
- Osmosis — movement of water across a semi-permeable membrane to balance solute concentration.
Action Items / Next Steps
- Review the 60-40-20 rule for body fluid distribution.
- Practice calculating ICF, ECF, and blood volumes using provided formulas.
- Study the indicator-dilution principle and which indicators are used for each compartment.